Fast Radio Bursts

Eighteen years after their discovery, the astronomical sources and radiation mechanisms of fast radio bursts remain mysterious. Their radiation is as bright as that of pulsars, with brightness temperatures as high as $\sim 10^{36}$ K, implying coherent emission, but the plasma physics that forms the coherent charge bunches, with net charges of order a Coulomb, is not understood. Some FRB have been identified with galaxies at redshifts of a few tenths, but one originated within a globular cluster in the galaxy M81 at a distance of 3.6 Mpc. A minority of FRB have been observed to repeat, in some cases thousands of times. The vast majority of FRB have not been observed to repeat, but it is not known if they are truly ``one-offs'' or repeat at unobservably long intervals. Some FRB originate within dense, rapidly varying, plasma environments, while others appear to be surrounded by high vacuum. Hypotheses for their sources include magnetars and black hole accretion discs.